With the advent of modern antipollution laws in the United States and around the world, significant and new methods of minimizing various pollutants are being investigated. The burning of fuel, be the fuel wood, coal, oils, or natural gas, likely causes a majority of the pollution problems in existence today. Certain pollutants such as SO.sub.2, which are created as the result of the presence of a contaminant in the fuel source may be removed either by treating the fuel to remove the contaminant or by treating the exhaust gas eventually produced to remove the resulting pollutant. Other pollutants such as carbon monoxide, which are created as the result of incomplete combustion, may be removed by postcombustion oxidation or by improving the combustion process. The other principal pollutant, NO.sub.x (an equilibrium mixture mostly of NO, but also containing very minor amounts of NO.sub.2), may be dealt with either by controlling the combustion process to minimize its production or by later removal. Removal of NO.sub.x, once produced, is a difficult task because of its relative stability and its low concentrations in most exhaust gases. One ingenious solution used in automobiles is the use of carbon monoxide to reduce NO.sub.x to nitrogen while oxidizing the carbon monoxide to carbon dioxide. However, the need to react two pollutants also speaks to a conclusion that the initial combustion reaction was inefficient.
It must be observed that unlike the situation with sulfur pollutants where the sulfur contaminant may be removed from the fuel, removal of nitrogen from the air fed to the combustion process is an impractical solution. Unlike the situation with CO, improvement of the combustion reaction would likely increase the level of NO.sub.x produced due to the higher temperatures then involved.
Nevertheless, the challenge to reduce NO.sub.x remains and several different methods have been suggested. The process chosen must not substantially conflict with the goal for which the combustion gas was created, i.e., the recovery of its heat value in a turbine, boiler, or furnace.
Many recognize that a fruitful way to control NO.sub.x production is to limit the localized and bulk temperatures in the combustion zone to something less than 1800.degree. C. See, for instance, U.S. Pat. No. 4,731,989 to Furuya et al. at column 1, lines 52-59 and U.S. Pat. No. 4,088,435 to Hindin et al. at column 12.
There are a number of ways to control the temperature, such as by dilution with excess air, controlled oxidation using one or more catalysts, or staged combustion using variously lean or rich fuel mixtures. Combinations of these methods are also known.
One widely attempted method is the use of multistage catalytic combustors. Most of these processes utilize multi-section catalysts with metal oxide or ceramic catalyst carriers. Typical of such disclosures are:
__________________________________________________________________________ Country Document 1st Stage 2nd Stage 3rd Stage __________________________________________________________________________ Japan Kokai 60-205129 Pt-group/Al.sub.2 O.sub.3 & SiO.sub.2 La/SiO.sub.2.Al.sub.2 O.sub.3 Japan Kokai 60-147243 La & Pd & Pt/Al.sub.2 O.sub.3 ferrite/Al.sub.2 O.sub.3 Japan Kokai 60-66022 Pd & Pt/ZrO.sub.2 Ni/ZrO.sub.2 Japan Kokai 60-60424 Pd/-- CaO & Al.sub.2 O.sub.3 & NiO & w/noble metal Japan Kokai 60-51545 Pd/* Pt/* LaCoO.sub.3 /* Japan Kokai 60-51543 Pd/* Pt/* Japan Kokai 60-51544 Pd/* Pt/* base metal oxide/* Japan Kokai 60-54736 Pd/* Pt or Pt--Rh or Ni base metal oxide or LaCO.sub.3 /* Japan Kokai 60-202235 MoO.sub.4 /-- CoO.sub.3 & ZrO.sub.2 & noble metal Japan Kokai 60-200021 Pd & Al.sub.2 O.sub.3 /+* Pd & Al.sub.2 O.sub.3 /** Pt/** Japan Kokai 60-147243 noble metal/heat ferrite/heat resistant carrier resistant carrier Japan Kokai 60-60424 La or Nd/Al.sub.2 O.sub.3 0.5% SiO.sub.2 Pd or Pt/NiO & Al.sub.2 O.sub.3 & CaO 0.5% SiO Japan Kokai 60-14938 Pd/? Pt/? Japan Kokai 60-14939 Pd & Pt/refractory ? ? Japan Kokai 61-252409 Pd & Pt/*** Pd & Ni/*** Pd & Pt/*** Japan Kokai 62-080419 Pd & Pt Pd, Pt & NiO Pt or Pt & Pd Japan Kokai 62-080420 Pd & Pt & NiO Pt Pt & Pd Japan Kokai 63-080848 Pt & Pd Pd & Pt & NiO Pt or Pt & Pd Japan Kokai 63-080849 Pd, Pt, NiO/? Pd & Pt (or NiO)/? Pt or Pd & __________________________________________________________________________ Pt/? *alumina or zirconia on mullite or cordierite **Ce in first layer; one or more of Zr, Sr, Ba in second layer; at least one of La and Nd in third layer. ***monolithic support stabilized with lanthanide or alkaline earth metal oxide Note: the catalysts in this Table are characterized as "a"/"b" where "a" is the active metal and "b" is the carrier
The use of such ceramic or metal oxide supports is clearly well known. The structures formed do not readily melt or oxidize as would a metallic support. A ceramic support carefully designed for use in a particular temperature range can provide adequate service in that temperature range. Nevertheless, many such materials can undergo phase changes or react with other components of the catalyst system at temperatures above 1100.degree. C., e.g., the gamma phase alumina changes to alpha phase alumina in that temperature region. In addition, such ceramic substrates are fragile, subject to cracking and failure as a result of vibration, mechanical shock, or thermal shock. Thermal shock is a particular problem in catalytic combustors used in gas turbines. During startup and shutdown, large temperature gradients can develop in the catalyst leading to high mechanical stresses that result in cracking and fracture.
Typical of the efforts to improve the high temperature stability of the metal oxide or ceramic catalyst supports are the inclusion of an alkaline earth metal or lanthanide or additional metals into the support, often in combination with other physical treatment steps:
______________________________________ Country Document Assignee or Inventor ______________________________________ Japan Kokai 61-209044 (Babcock-Hitachi KK) Japan Kokai 61-216734 (Babcock-Hitachi KK) Japan Kokai 62-071535 (Babcock-Hitachi KK) Japan Kokai 62-001454 (Babcock-Hitachi KK) Japan Kokai 62-45343 (Babcock-Hitachi KK) Japan Kokai 62-289237 (Babcock-Hitachi KK) Japan Kokai 62-221445 (Babcock-Hitachi KK) U.S. Pat. No. 4,793,797 (Kato et al.) U.S. Pat. No. 4,220,559 (Polinski et al.) U.S. Pat. No. 3,870,455 (Hindin) U.S. Pat. No. 4,711,872 (Kato et al.) ______________________________________
However, even with the inclusion of such high temperature stability improvements, ceramics columnar catalyst with holes in the column walls to promote equal distribution of fuel gas and temperature amongst the columns lest cracks appear.
High temperatures (above 1100.degree. C.) are also detrimental to the catalytic layer resulting in surface area loss, vaporization of metal catalysts, and reaction of catalytic components with the ceramic catalyst components to form less active or inactive substances.
Of the numerous catalysts disclosed in the combustion literature may be found the platinum group metals, platinum, palladium, ruthenium, iridium, and rhodium; sometimes alone, sometimes in mixtures with other members of the group, sometimes with non-platinum group promoters or co-catalysts.
Other combustion catalysts include metallic oxides, particularly Group VIII and Group I metal oxides. For instance, in an article by Kaiji et al, COMPLETE OXIDATION OF METHANE OVER PEROVSKITE OXIDES, Catalysis Letters I (1988) 299-306, J. C. Baltzer A. G. Scientific Publishing Co., the authors describe a set of perovskite oxide catalysts suitable for the oxidation of methane which are generically described as ABO.sub.3, particularly oxides formulated as La.sub.1-x Me.sub.x MnO.sub.3, where Me denotes Ca, Sr, or Ba.
Similarly, a number of articles by a group associated with Kyushu University described combustion catalysts based on BaO.6Al.sub.2 O.sub.3 :
1. PREPARATION AND CHARACTERIZATION OF LARGE SURFACE AREA BaO.6Al.sub.2 O.sub.3, Machida et al, Bull. Chem. Soc. Jpn., 61,3659-3665 (1988), PA0 2. HIGH TEMPERATURE CATALYTIC COMBUSTION OVER CATION-SUBSTITUTED BARIUM HEXAALUMINATES, Machida et al, Chemistry Letters, 767-770, 1987, PA0 3. ANALYTICAL ELECTRON MICROSCOPE ANALYSIS OF THE FORMATION OF BaO.6Al.sub.2 O.sub.3, Machida et al, J. Am. Ceram. Soc, 71 (12) 1142-47 (1988), PA0 4. EFFECT OF ADDITIVES ON THE SURFACE AREA OF OXIDE SUPPORTS FOR CATALYTIC COMBUSTION, J. Cat. 103, 385-393 (1987), and PA0 5. SURFACE AREAS AND CATALYTIC ACTIVITIES OF Mn-SUBSTITUTED HEXAALUMINATES WITH VARIOUS CATION COMPOSITIONS IN THE MIRROR PLANE, Chem. Lett., 1461-1464, 1988.
Similarly, U.S. Pat. No. 4,788,174, to Arai, suggests a heat resistant catalyst suitable for catalytic combustion having the formula A.sub.1-x C.sub.x B.sub.x Al.sub.12-y O.sub.19-a, where A is at least one element selected from Ca, Ba, and Sr; C is K and/or Rb; B is at least one from Mn, Co, Fe, Ni, Cu, and Cr; z is a value in the range from 0-0.4; x is a value in the range of 0.1-4, y is a value in the range of about x-2x; a is a value determined by the valence X, Y, and Z of the respective element A, C, and B and the value of x, y, and z and it is expressed as a=1.5{X-z(X-Y)+xZ-3y}.
In addition to the strictly catalytic combustion processes, certain processes use a final step in which remaining combustibles are homogeneously combusted prior to recovering the heat from the gas.
A number of the three stage catalyst combination systems discussed above also have post-combustion steps. For instance, a series of Japanese Kokai assigned to Nippon Shokubai Kagaku (62-080419, 62-080420, 63-080847, 63-080848, and 63-080849) disclose three stages of catalytic combustion followed by a secondary combustion step. As was noted above, the catalysts used in these processes are quite different from the catalysts used in the inventive process. Additionally, these Kokai suggest that in the use of a post-combustion step, the resulting gas temperature is said to reach only "750.degree. C. to 1100.degree. C.". In clear contrast, the inventive process may be seen to reach substantially higher temperatures depending upon the makeup of the fuel/air mixture.
Other combustion catalyst/post-catalyst homogeneous combustion processes are known. European Patent Application 0,198,948 (also issued to NSK) shows a two or three stage catalytic process followed by a post-combustion step. The temperature of the post-combusted gas was said to reach 1300.degree. C. with an outlet temperature from the catalyst (approximately the bulk gas phase temperature) of 900.degree. C. The catalyst structures disclosed in the NSK Kokai are not, however, protected from the deleterious effects of the combustion taking place within the catalytic zones and consequently the supports will deteriorate.
The patent to Furuya et al. (U.S. Pat. No. 4,731,989) discloses a single stage catalyst with injection of additional fuel followed by post-catalyst combustion.
An aspect in the practice of our inventive process is the use of metal integral heat exchange structures in the latter catalytic stage or stages of the combustion. Generically, the concept is to position a catalyst layer on one surface of a wall in the catalytic structure which is opposite a surface having no catalyst. Both sides are in contact with the flowing fuel-gas mixture: on one side reactive heat is produced; on the other side that reactive heat is transferred to the flowing gas.
Structures having an integral heat exchange feature are shown in Japanese Kokai 59-136, 140 and 61-259,013. In addition to a number of other differences, the structures are disclosed to be used in isolation and not in conjunction with other catalyst stages. Additionally, the staged use of the structure with different catalytic metals is not shown in the two Kokai.
None of the processes in this discussion appear to show a combination two stage catalyst system in which the catalyst supports are metallic, in which the catalysts are specifically varied to utilize their particular benefits, in which integral heat exchange is selectively applied to control combustion temperature, and particularly, in which high gas temperatures are achieved while maintaining low NO.sub.x production and catalyst temperatures.